To revert to his original appearance, he had to place himself in a position that reflected the moon.
What better place than the stream at a distance from the cottage that he and Reina had visited a couple of times during their time together?
Therefore, he had escaped from her embrace once she fell asleep and made his way towards the stream which would act as the means for him to shift his appearance back to what he originally possessed.
When he had arrived at the stream, he had walked into the freezing water and had submerged himself under the surface of the stream to enhance the properties of the spell he had cast on himself.
His powers hadn't returned to their fullest degree which had led him to use these additional elements to boost the spell he had to cast in order to get back into his original appearance.
Once underneath the surface of the water, he stopped moving entirely which caused his cat-like body to drown to the bottom of the stream.
Bye, Little Overlord!